The present invention relates, in general, to pipe organs, and more particularly to a system for electronically positioning the volume-controlling shutters for a pipe organ swell chamber.
In pipe organs the various tones are derived from wind blown pipes. Each pipe is capable of sounding at only a single intensity (volume) level; and if an attempt is made to control the loudness, as for example by changing the air pressure, an unacceptable variation in tuning will result. In order to surmount this limitation, it is customary in a pipe organ to place the pipes of at least some of the divisions of the organ in a box or room called a (swell box or swell chamber) separated from the main listening room by a set of moveable shutters (sometimes called shades). When closed, these shutters effectively close off the chamber from the listening room and substantially attenuate the sound that is heard. A swell pedal (or swell shoe) is provided at the console and can be adjusted to several positions for controlling the extent that the shutters are opened. Patents that relate to swell shutters and swell boxes include U.S. Pat. Nos. 500,040 issued to E. M. Skinner in 1893, and 2,005,643 issued to H. Willis et al. in 1935.
In early swell devices the shutters were mechanically linked to the swell pedal by means of operating rods or cables. With the development of electrically controlled key actions it became common to place the console at a remote location from the chambers, and it became difficult to arrange the necessary mechanical linkage to the swell chamber shutters. The next step was the development of electrically controlled pneumatic swell shutter operators, an example of which is shown in U.S. Pat. No. 2,072,844. Such pneumatic devices have been employed for decades and have been reasonably successful; however they have many limitations, the most important of which is their large size. Pipe organs operate on low wind pressures, and in order to achieve sufficient force to move the relatively heavy shutters with promptness, operating pneumatics of great bulk are necessary. In past decades wind pressures of up to 15 inches water gauge were commonly used. This is in fact a relatively low pressure (about one-half pound per square inch), and created problems in driving prior swell shade operators. However, the problem of rapidly and accurately moving swell shade operators is now greatly exacerbated by the current trend back to the very low pressures used in early organs, where the pipes speak on pressures in the order of one and one-half (11/2) or two (2) inches water gauge.
Attempts have been made to make electric motor-driven swell shutter operators, but none have been very successful because of one or more of the limitations of cost, size or lack of precision in operation. An example of such an operation is found in U.S. Pat. No. 3,701,833. A successful swell operator must move the shutters promptly, but precisely, without hunting, and without slamming as the shutters close. In addition it must be easily adaptable to a wide range of shutter sizes and weights, and must be easily adjustable to produce a smooth swell, whereby the loudness of sound heard varies uniformly from soft to loud as the swell shoe is moved from closed to open.